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Trace-Element Geochemistry of Diamond-Hosted Olivine Inclusions

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Trace-Element Geochemistry of Diamond-Hosted Olivine Inclusions Edinburgh Research Explorer Trace-element geochemistry of diamond-hosted olivine inclusions from the Akwatia Mine, West African Craton: implications for diamond paragenesis and geothermobarometry Citation for published version: De Hoog, C-J, Stachel, T & Harris, JW 2019, 'Trace-element geochemistry of diamond-hosted olivine inclusions from the Akwatia Mine, West African Craton: implications for diamond paragenesis and geothermobarometry', Contributions to Mineralogy and Petrology, vol. 174, 100. https://doi.org/10.1007/s00410-019-1634-y Digital Object Identifier (DOI): 10.1007/s00410-019-1634-y Link: Link to publication record in Edinburgh Research Explorer Document Version: Publisher's PDF, also known as Version of record Published In: Contributions to Mineralogy and Petrology Publisher Rights Statement: © The Author(s) 2019 This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. General rights Copyright for the publications made accessible via the Edinburgh Research Explorer is retained by the author(s) and / or other copyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associated with these rights. Take down policy The University of Edinburgh has made every reasonable effort to ensure that Edinburgh Research Explorer content complies with UK legislation. If you believe that the public display of this file breaches copyright please contact [email protected] providing details, and we will remove access to the work immediately and investigate your claim. Download date: 28. Sep. 2021 Contributions to Mineralogy and Petrology (2019) 174:100 https://doi.org/10.1007/s00410-019-1634-y ORIGINAL PAPER Trace‑element geochemistry of diamond‑hosted olivine inclusions from the Akwatia Mine, West African Craton: implications for diamond paragenesis and geothermobarometry J. C. M. De Hoog1 · T. Stachel2 · J. W. Harris3 Received: 8 May 2019 / Accepted: 16 October 2019 / Published online: 14 November 2019 © The Author(s) 2019 Abstract Trace-element concentrations in olivine and coexisting garnets included in diamonds from the Akwatia Mine (Ghana, West African Craton) were measured to show that olivine can provide similar information about equilibration temperature, diamond paragenesis and mantle processes as garnet. Trace-element systematics can be used to distinguish harzburgitic olivines from lherzolite ones: if Ca/Al ratios of olivine are below the mantle lherzolite trend (Ca/Al < 2.2), they are derived from a harz- burgitic mantle source, and syngenetic garnets are without exception subcalcic G10 garnets. For harzburgitic olivines that cannot be identifed this way, Na and Ca contents can be used: olivine inclusions with < 60 µg/g Na and Na/Al < 0.7 are all harzburgitic, whereas those with > 300 µg/g Ca or > 60 µg/g Na are lherzolitic. Conventional geothermobarometry indicates that Akwatia diamonds formed and resided close to a 39 mW/m2 conductive geotherm. A similar value can be derived from Al in olivine geothermometry, with TAl-ol ranging from 1020 to 1325 °C. Ni in garnet temperatures is on average somewhat higher (TNi-grt = 1115–1335 °C) and the correlation between the two thermometers is weak, which may be not only due to the large uncertainties in the calibrations, but also due to disequilibrium between inclusions from the same diamond. Calcium in olivine should not be used as a geothermobarometer for harzburgitic olivines, and often gives unrealistic P–T estimates for lherzolitic olivine as well. Diamond-hosted olivine inclusions indicate growth in an extremely depleted (low Ti, Ca, Na, high Cr#) environment with no residual clinopyroxene. They are distinct from olivines from mantle xenoliths which show higher, more variable Ti contents and lower Cr#. Hence, most olivine inclusions in Akwatia diamonds escaped the referti- lisation processes that have afected most mantle xenoliths. Lherzolitic inclusions are probably the result of refertilisation after undergoing high-degree melting frst. Trivalent cations appear to behave diferently in harzburgitic diamond-hosted olivine inclusions than lherzolitic inclusions and olivine from mantle xenoliths. Some divalent chromium is predicted to be present in most olivine inclusions, which may explain high concentrations up to 0.16 wt% Cr2O3 observed in some diamond inclusions. Strong heterogeneity of Cr, V and Al in several inclusions may also result in apparent high Cr contents, and is probably due to late-stage processes during exhumation. However, in general, diamond-hosted olivine inclusions have lower Cr and V than expected compared to mantle xenoliths. Reduced Na activity in depleted harzburgites limits the uptake of Cr, V and Sc via Na–M3+ exchange. In contrast, Al partitioning in harzburgites is not signifcantly reduced compared to lherzolites, presumably due to uptake of Al in olivine by Al–Al exchange. Keywords Diamond inclusions · Olivine · Geothermobarometry · Mantle petrology Communicated by Daniela Rubatto. Electronic supplementary material The online version of this article (https ://doi.org/10.1007/s0041 0-019-1634-y) contains supplementary material, which is available to authorized users. * J. C. M. De Hoog 2 Department of Earth and Atmospheric Sciences, University [email protected] of Alberta, Edmonton, AB T6G 2E3, Canada 3 School of Geographical and Earth Sciences, University 1 School of GeoSciences, The University of Edinburgh, Grant of Glasgow, Gregory Building, Glasgow G12 8QQ, UK Institute, James Hutton Road, Edinburgh EH9 3FE, UK Vol.:(0123456789)1 3 100 Page 2 of 28 Contributions to Mineralogy and Petrology (2019) 174:100 Introduction formation. These conditions can be estimated based on the chemical composition of co-existing minerals within the The subcontinental lithospheric mantle (SCLM) beneath diamond, assuming that they were trapped simultaneously cratons has been studied extensively as it is a key source (i.e., syngenetic) or at least under similar chemical and of information about the geological and tectonic history of P–T conditions. The requirement of equilibrium between the early Earth. As the main source of diamonds, it is also co-existing minerals does not apply to single-element ther- an important economic reservoir. Olivine is a common mometers, such as single cpx thermobarometry (Nimis and inclusion in such diamonds (Sobolev et al. 2008; Stachel Taylor 2000), Al in olivine thermometry (De Hoog et al. and Harris 2008), as well as the most common mineral in 2010) or Ni in garnet (Canil 1996; Ryan et al. 1996), as Earth’s upper mantle, and may as such be an important tool these are assumed to be in equilibrium with a relatively in elucidating mantle and diamond formation processes. constant source. However, few, if any, studies have evalu- However, due to its low trace element contents and the ated the validity of olivine-based single-element thermom- lack of an empirical framework to interpret olivine data, eters for diamond-hosted inclusions. its full potential has not been explored (De Hoog et al. A further source of uncertainty in interpreting trace ele- 2010; Foley et al. 2013) and it is rarely used as an indica- ment data is the infuence of the chemical environment on tor mineral for diamond prospecting (e.g., Shchukina and trace element partitioning. For example, diferent exchange Shchukin 2018). mechanisms appear to operate for Cr and Al in olivine in Inclusions in diamonds are of particular interest spinel vs. garnet peridotites (De Hoog et al. 2010; Witt-Eick- because diamonds are robust containers shielding inclu- schen and O’Neill 2005). As Na may act to charge balance sions from secular processes in the Earth’s mantle, and trivalent cations (De Hoog et al. 2010; Hervig et al. 1986), thus provide a window into early mantle processes that the absence of clinopyroxene in harzburgitic lithologies have been overprinted in mantle xenolith suites (Stachel prevalent in the mantle source of diamonds may afect trace and Harris 2008). For example, it has been noted that gar- element distributions and its use in tracing mantle processes net inclusions in diamonds contain a much larger frac- and geothermometry. tion of Ca-depleted garnets than mantle xenoliths from This study aims to evaluate: diamond paragenesis based the same locality, and that olivine inclusions in diamonds on the trace element compositions of olivine inclusions, the can have much higher Cr2O3 contents (up to 0.5 wt%) than validity of trace element geothermometry, and the use of commonly recorded in olivine from mantle xenoliths (up olivine to study mantle processes. The diamond suite used is to 0.1 wt%; De Hoog et al. 2010; Hervig et al. 1980b; from Akwatia, Ghana, in which olivine is a common inclu- Sobolev et al. 2009; Stachel and Harris 2008). sion and frequently co-exists with garnet (Stachel and Harris Diamond formation is associated with garnet-bearing sec- 1997a, b). It is, therefore, an ideal suite for the purpose of tions of the SCLM in which harzburgites predominate over this study. lherzolites (Gurney et al. 1993). Harzburgitic assemblages with strongly Ca-depleted garnets (G10 garnets; Dawson and Stephens 1975; Grütter et al. 2004) are of particular inter-
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